Method of Making an Udersea Pipe, the Method Including Peening Assembly Welds Inside the Pipe

Abstract

A method of making undersea steel pipes, by assembling unit pipe elements together end-to-end by welding, the steel or metal alloy weld beads of said welds being disposed on the outside of the pipe, wherein localized peening is performed on the inside of the pipe to increase the compression of the steel or metal in the vicinity of the welds and over the adjacent peripheral inside surface of the pipe on either side of the weld so as to create a swath of surface that has been peened over a limited distance L in the axial longitudinal direction of said pipe. The peening is preferably performed using a wheeled carriage fitted with a peening tool.

Description

The present invention relates to a method of making undersea pipes for conveying corrosive fluids, and in particular water, the method comprising assembling unit pipe elements together by welding.

The present invention relates more particularly to a subsurface connection installation between a floating support and an oil loading buoy.

The present invention relates more particularly to a bottom-surface connection installation comprising at least one undersea pipe providing a connection between a floating support and the bottom of the sea, in particular at great depth. Such undersea pipes are referred to as “risers” and they are made up of unit tubular elements made of steel that are welded together end-to-end.

More particularly, the present invention provides a riser type undersea pipe for making a connection between a floating support and the bottom of the sea, said riser being constituted by a rigid, catenary-type pipe that extends from said floating support to a point of contact with the sea bottom.

The technical field of the invention is thus the field of fabricating and installing undersea pipes and more particularly production bottom-surface connections for offshore extraction of oil, gas, or other soluble or phase-change material, or a suspension of mineral material, from an undersea well head in order to develop production fields located at sea or off-shore. The main and immediate application of the invention lies in the field of oil production, and also in reinjecting water and producing or reinjecting gas.

In general, a floating support includes anchor means enabling it to remain in position in spite of the effects of currents, winds, and swell. It also generally includes means for drilling, storing, and processing oil, and means for off-loading to off-loading tankers that call at regular intervals to remove production. Such floating supports are referred to as floating production storage off-loading (FPSO) vessels or as “floating drilling and production units” (FPDU) when the floating support is also used for performing drilling operations with wells being deflected in the depth of the water.

An undersea pipe or “riser” of the invention may constitute either a “production pipe” for crude oil or gas, or a water injection pipe providing a connection with an undersea well head at the sea bottom, or indeed a “drilling riser” providing the connection between the floating support and a well head located on the sea bottom.

A multiplicity of lines are generally installed on FPSOs and it is necessary to implement either hybrid-tower type bottom-surface connections or else catenary type connections, i.e. connections that follow a catenary curve.

When the bottom-surface connection pipe is of the catenary type, it provides a direct connection between a floating support and a point of contact with the sea bottom that is offset from the axis of said support, said pipe taking up a so-called “catenary” configuration under the effect of its own weight, i.e. a curve having a radius of curvature that decreases from the surface down to the point of contact with the sea bottom, with the axis of said pipe forming an angle α relative to the vertical that varies in general from 10° to 20° at the level of the floating support up to, theoretically, 90° at the sea bottom corresponding to a theoretical position that is substantially tangential to the horizontal, as explained below.

Catenary type connections are generally made with the help of flexible pipes, however they are extremely expensive because of the complex structure of the pipe.

As a result, substantially vertical risers have been developed so as to bring the catenary-configuration flexible connection closer to the surface near the floating support, thus making it possible to minimize the length of said flexible pipe, and also to minimize the forces that are applied thereto, thereby considerably reducing its cost.

Once the depth of water reaches or exceeds 800 meters (m) to 1000 m, it becomes possible to make said bottom-surface connection with the help of a thick-walled rigid pipe since the considerable length of the pipe presents sufficient flexibility to obtain a satisfactory catenary configuration while remaining within acceptable stress limits.

Such rigid risers of thick strong materials in a catenary configuration are commonly referred to as steel catenary risers (SCRs) regardless of whether they are made of steel or of some other material such as a composite material.

Such “SCRs” are much simpler to make than flexible pipes and therefore much less expensive.

The geometrical curve formed by a pipe of uniform weight in suspension and subjected to gravity, known as a “catenary”, is a mathematical function of the hyperbolic cosine type (Cos h(x)=(e^x+e^−x)/2), relating the abscissa and the ordinate of an arbitrary point on the curve in application of the following formulae:

y=R₀(cos h(x/R₀)−1)

R=R₀(Y/R₀+1)²

in which:

• • x is the distance in the horizontal direction between the point of contact and a point M on the curve;
  • y represents the altitude of point M (x and y are thus the abscissa and the ordinate of a point M on the curve relative to an orthogonal frame of reference having its origin at said point of contact);
  • R₀ is the radius of curvature at said point of contact, i.e. the point where the tangent is horizontal; and
  • R is the radius of curvature at the point M (x,y).

Thus, curvature varies along the catenary from the surface where its radius of curvature has a maximum value R_max down to the point of contact where its radius of curvature has a minimum value R_min (or R₀ in the above formula). Under the effect of waves, wind, and current, the surface support moves laterally and vertically, thereby having the effect of raising and lowering the catenary-shaped pipe in the vicinity of the sea bottom.

Thus, the pipe presents a radius of curvature that is greatest at the top of the catenary, and in generally at least 1500 m, and in particular lies in the range 1500 m to 5000 m, i.e. at the point where it suspended from the FPSO, with said radius of curvature decreasing down to the point of contact with the bottom. At that location, the radius of curvature is at a minimum in the portion that is suspended. However, in the adjacent portion that is resting on the sea bottom, said pipe is theoretically in a straight line so its radius of curvature is theoretically infinite. In fact, since some residual curvature remains, said radius is not infinite, but it is extremely large.

Thus, as the floating support moves on the surface, the point of contact moves forwards and backwards, and in the region that is lifted from or lowered onto the bottom, the radius of curvature passes in succession from a minimum value R_min to a value that is extremely large, or even infinite in a theoretical configuration where the undersea pipe rests on the sea bottom substantially in a straight line.

This alternating flexing gives rise to fatigue phenomena that are concentrated throughout the foot region of the catenary, and as a result the lifetime of such a pipe is greatly reduced and in general not compatible with the lifetimes desired for bottom-surface connections, i.e. 20 to 25 years, or even more.

In addition, during these alternating movements of the point of contact, it is observed that the stiffness of the pipe associated with the above-mentioned residual curvature acts over time to dig a furrow over the entire length that is raised and then lowered back down again, so as to create a transition region in which there exists a point of inflection where the radius of curvature, which is at a minimum at the foot of the catenary, changes direction in said transition region and increases finally to reach an infinite value in a portion of undersea pipe that is resting in a straight line on the sea bottom.

These repeated movements over long periods dig a furrow of considerable depth in bottoms that are poorly consolidated, as are commonly to be found at great depths, thereby having the effects of modifying the curvature of the catenary and, if the phenomenon becomes amplified, of leading to risks of damage to the pipes, or to other undersea pipes lying on the sea bottom, or to the SCRs that provide connections between said undersea pipes resting on the sea bottom and the surface.

These pipes are made by welding unit pipe elements together end-to-end. The unit pipe elements are themselves assembled to form strings, in general strings of two to four unit elements welded end-to-end, which strings are then taken to sea. In known manner, these strings are assembled by being welded to one another at sea from a pipe-laying ship, in particular in a J-lay tower. The assembly welds are made preferably and for the most part from the outside of the pipe.

The most critical portion of a riser is situated at the assembly welds between unit pipe elements, in particular in the portion of the riser that is closest to the point of contact, and the major fraction of the forces in this low portion of the catenary are generated by the movements of the floating support and by the excitations that are applied to the top portion of the catenary, which is subjected to current and swell, with all of these excitations then propagating mechanically along the entire length of the pipe to the foot of the catenary.

The steels from which pipes are made are selected to withstand fatigue throughout the lifetime of installations, however, the welds between pipe elements, in this catenary foot region constitute weak points when said pipe conveys water or fluid that includes water, and more particularly salt water. In the presence of water, said welds are subjected to fatigue and corrosion phenomena that give rise over time, and under varying bending stresses, to cracks that lead to said pipes being destroyed.

To mitigate that problem, welds are made between pipe elements using a stainless steel or an alloy that withstands corrosion. Anti-corrosion alloys are well known to the person skilled in the art, and are constituted mainly by nickel-based alloys, in particular of the Inconel type, preferably of a specific grade, and in particular Inconel 625 or 825. Such Inconels also present excellent resistance to fatigue as a result of their high elastic limits, thereby making it possible to achieve lifetimes of 20 to 30 years.

In order to enable the welds to be strong and to be made under good conditions, proposals have been made to line the insides of two pipe elements for welding together with the same stainless steel or corrosion-resistant alloy over a length of a few centimeters in the vicinity of each end of the pipe elements for welding together, so that the penetration pass of the weld that constitutes the future wall in contact with the fluid is of the same metal as the welding filler metal, and in particular Inconel. Such a lining of stainless steel or anti-corrosion alloy, in particular of the Inconel type, is provided using an expensive arc method referred to as “cladding”, and generally performed using a tungsten inert gas (TIG) method or a plasma method, associated with a filler wire or with a powder of stainless steel or of corrosion-resistant alloy.

The object of the present invention is to provide a novel method of fabricating and installing undersea pipes for conveying corrosive fluids and in particular water, the method comprising welding together undersea pipe strings at sea on board a ship for laying undersea pipes, which method should:

• • be reliable in terms of resistance to fatigue at each of the welds, and in particular avoid cracks appearing over time;
  • have as little effect as possible on the mechanical strength of the pipe and/or increase as little as possible head losses in the fluid conveyed inside the pipe when in operation; and
  • be as simple and as inexpensive as possible to implement, in particular with the assembly steps and in particular the welding, being performed as little as possible on board the laying ship.

In the present invention, the inventors have discovered that incipient cracks are located on the inside of the pipe in the vicinity of the small projection of the weld bead that extends towards the inside of the pipe, and not on the outside face comprising the main bulk of the weld bead on the outside of the pipe. More precisely, and as explained in the detailed description below given with reference to FIGS. 3E and 3F, the inventors have discovered that the origin of weld destruction lies in the transition region between the welds and the inside surface made of the base steel of the adjacent pipe, in which region traction stresses associated with thermal shocks during welding can give rise to physical defects, and in particular to incipient cracks located in said zone.

During welding, uncontrollable localized quenching or shrinkage occurs, leading to contraction stress states of the metal that are localized in and close to the weld region, even though the remainder of the adjacent surface of the pipe is either at rest or in compression.

In general, these problems of localized contraction stress in welds are solved by annealing to relax the stress. Other means are known for treating such problems in welds in order to relax traction stress, but they are not compatible with the time constraints and the desired rates of laying at sea. However, in present circumstances, annealing treatments are not possible for the welds made between undersea pipe elements while laying the pipe at sea.

The present invention provides a method of making steel undersea pipes for conveying corrosive fluids and in particular water, the method comprising assembling unit pipe elements together end-to-end, the weld beads of steel or metal alloy of said welds being located on the outside of the pipe, the method being characterized in that localized peening is performed inside the pipe to increase the level of compression stress in the steel or the metal in the vicinity of said welds and in the adjacent peripheral inside surface of the pipe on either side of the welds so as to create a peened surface swath that is peened over a distance L that is limited in the axial longitudinal direction XX of said pipe, i.e. over a fraction only of the length of each of the two pipe elements that are assembled together by said welds, and as measured from their respective abutting ends.

More particularly, said peened swath extends over a distance L that is not less than half the thickness of the pipe wall, and more preferably over a distance L that is less than twice the thickness of the pipe.

More particularly, the weld comprises a main weld bead on the outside of the pipe and a projection or seam on the inside that is of smaller thickness and that projects into the inside of the pipe.

This internal projection or seam results from the partial melting of the ends of the unit elements that are assembled together by welding, said melting taking place during the welding heat treatment.

More particularly, said peened swath extends over a distance L corresponding to the width of the weld on the inside of the pipe, in particular the width of said internal seam, which seam presents a width lying in practice in the range 3 millimeters (mm) to 5 mm, plus a width on either side lying in the range 1 mm to 10 mm, so as to give a distance L lying in the range 5 mm to 25 mm.

The term “peening” is used herein to mean surface treatment by multiple impacts using one or more projectiles so as to increase the level of compression stress in a region of the surface under treatment.

According to the present invention, it is the entire surface of said swath, i.e. the cylindrical inside surface section on either side of the weld, overlapping the weld, that is subjected to these impacts, with no region of the surface outside said swath being subjected to such an impact.

The projectiles may be in the form of balls or the tips of pointed spindles, the projectiles striking the surface for treatment at their ends and, during impacts, the kinetic energy of the projectiles is transformed into plastic and elastic deformation energy in the surface being treated, thereby having the effect of increasing the compression stress in the material where it is treated, and as a result eliminating residual regions of traction stress.

Peening tools that can be used in the present invention are described in FR 2 791 293 in the name of one of the Applicants, however more rudimentary peening tools as described for example in U.S. Pat. No. 3,937,055 could also be used.

Peening in the present invention consists, so to speak, in cold forging to eliminate residual traction stresses by deforming the material in the peened surface. It should be observed that it is not desired to eliminate any extra thickness associated with an inside seam or projection of the weld bead, but only to apply compression in substantially uniform manner to the surface of the welding region and of the adjacent regions, using sufficient energy to plasticize and deform the metal so as to eliminate any residual traction stresses due to the welding operation.

Still more particularly, said ends of the unit pipe elements for welding together comprise, in longitudinal axial section, a straight end beside the inside of the pipe forming a root face that preferably occupies at least one-fourth of the thickness of the main portion of the pipe and that is extended towards the outside of the pipe by a sloping chamfer.

Under such circumstances, the inside projection or seam of the weld that stands proud is a made up molten metal from said root face and of the filler metal.

It will be understood that said chamfer faces towards the outside of the pipe so that it can receive a weld bead deposited between the two chamfers at the ends of two abutting pipe elements, thereby substantially forming a V-shape at the end of the two pipe elements for butt welding together.

In an advantageous implementation, material is removed by prior machining, preferably by grinding or by milling, from the inside surface of the pipe and from the weld bead over the surface that is to be peened, prior to said peening.

Also advantageously, said peening is performed at least in the transition region between the inside surface of the weld bead and the adjacent inside surface of the pipe.

More particularly, said peening is performed in such a manner as to establish compression or to increase compression over a thickness of 0.2 mm to 2 mm of said inside surface of the pipe and of said weld.

In one implementation, the limited distance L represents one to three times the thickness of the pipe.

Still more particularly, peening is performed in such a manner as to obtain compression stress that is greater than 5 megapascals (MPa), preferably greater than 50 MPa, in particular lying in the range 50 MPa to 1000 MPa, over the entire peened surface.

In a preferred implementation, said peening is performed with a peening device that is moved inside said pipe in translation and in rotation in the vicinity of said weld, the peening device comprising:

• • at least one peening tool mounted on a first motor-driven carriage;
  • said first carriage being suitable for moving inside said pipe in translation in the axial longitudinal direction XX of said pipe;
  • said first carriage supporting means for moving said peening tool in radial translation YY relative to said first carriage, enabling said peening tool to be applied against the inside surface of the pipe, or enabling the peening tool to be set back away from said inside surface of the pipe; and
  • means for moving said peening tool in rotation about said axial longitudinal axis XX of the pipe, enabling said peening to be performed over the entire circumference of the inside surface of said pipe one said rotation of the peening tool.

In a particular implementation, said peening tool comprises a vibratory surface that preferably extends over said limited distance L in the axial longitudinal direction XX, and a plurality of projectiles of rounded or pointed type suitable for being projected towards the inside surface for treatment by said vibrating surface in order to create a plurality of impacts.

In a preferred implementation, the method of the invention is characterized in that said first carriage includes means for moving said peening tool in translation relative to said first carriage in said longitudinal axial direction XX.

In particular, when the peening tool has a plurality of projectiles that are projected radially from a vibration surface, itself extending over at least some distance in the longitudinal direction, said means for imparting relative movement in longitudinal translation to the peening tool are suitable for moving said projectiles over at least a distance corresponding to the spacing between two successive projectiles so that the entire treated surface can be peened completely in substantially uniform manner.

Still more particularly, the method of the invention is characterized in that it includes the following steps:

• • said first carriage is moved in translation inside said pipe in said axial direction XX, such that said peening tool is substantially positioned so that it can perform peening in said weld region and on either side thereof over a said distance L for peening, in said longitudinal axial direction XX and astride said weld; then
  • said peening tool is moved against or close to the inside surface of the pipe by moving said peening tool in radial translation YY; then
  • said peening tool is then moved in rotation about said axial longitudinal axis XX over the circumference of the inside surface of the pipe; and then where appropriate, the peening tool is moved in translation in the axial longitudinal direction XX relative to said first carriage so as to perform the peening and compression over the entire peened surface, in particular for a peening tool including a plurality of said projectiles of pointed or rounded type that are spaced about from one another.

It will be understood that the longitudinal movement of the peening tool in translation relative to said first carriage may be performed either continuously, or else essentially between two of said rotations of said peening tool. This makes it possible to avoid leaving any non-peened area between two impact regions of said successive projectiles, and thus to reach the most critical regions that are situated at the interface between the seam of the weld bead and the base metal of the pipe.

According to other characteristics that are advantageous:

• • said first motor-driven carriage supports a first shaft placed in said axial longitudinal direction XX; and
  • said first shaft supports a transverse guide support suitable for guiding the movement of a second carriage in radial translation in a transverse direction perpendicular to said axial longitudinal direction XX, and comprising means suitable for keeping said peening tool in position facing the inside surface of said pipe; and
  • said first shaft comprises means for driving it in controlled rotation about its said axial longitudinal axis XX, so as to enable said peening tool to be moved over the circumference of the inside surface of the pipe in controlled rotation about its axis; and
  • said first shaft is preferably suitable for being driven in translation relative to said first carriage in said axial longitudinal direction XX, in particular over at least a short distance δx corresponding to a fraction of the distance between two successive projectiles if any, or over a distance lying in the range 0.1 mm to 10 mm when using a peening tool having a single row of projectiles.

In a particular embodiment, said peening tool has a plurality of projectiles, in particular of the rounded or pointed type that are projected from a vibrating surface of said peening tool against said surface for peening, in particular in a radial direction, where appropriate.

Nevertheless, in a particular embodiment, said peening tool is mounted to pivot relative to said second carriage, thus enabling the angle of inclination 3 of the projection direction Y₁Y₁ of said projectiles to be varied relative to said direction YY of movement in radial translation of said second carriage.

This embodiment makes it possible to optimize peening of the transition regions between the inside weld seam and the adjacent pipe wall, in particular when there is no prior machining of said inside weld seam.

In the method in which prior grinding is performed, the prior grinding of the surface for peening is performed with a grinder tool having a rotary grindwheel mounted in the place of or together with a said peening tool on a said first carriage.

More particularly, said welding is performed using carbon steel, stainless steel, or a corrosion-resisting alloy of the Inconel type having high elasticity, and good fatigue resistance, and preferably Inconel of grade 625 or 825.

Still more particularly, the method of the invention comprises the following successive steps:

1) in a workshop on land, assembling the respective ends of at least two unit pipe elements together end-to-end by said welding in order to form pipe strings; and

2) at sea, on board a laying ship fitted with a J-lay tower, assembling respective ends of said strings together by said welding to form a pipe.

The present invention also provides a bottom-surface connection undersea pipe having at least a portion including regions of said assembly welds between unit pipe elements that have been put into compression by a method of the invention.

More particularly, the present invention provides a bottom-surface connection undersea pipe of the invention that is characterized in that it is a catenary pipe of the SCR type with at least a portion thereof, including the region that comes into contact with the bottom and extending from the bottom over at least 100 m, and preferably 200 m, being assembled by a pipe-making method of the invention.

Finally, the present invention provides a peening device comprising at least a said peening tool mounted on a said first carriage suitable for moving in translation inside a pipe, the device comprising a said peening tool suitable for moving in longitudinal translation XX relative to said first carriage and in rotation about said axial longitudinal axis XX of the pipe in the vicinity of said welds, as defined above.

Other characteristics and advantages of the present invention appear in the detailed light of embodiments described below with reference to the accompanying figures, in which:

FIG. 1 is a side view of a pipe in a simple catenary configuration 1, suspended from a floating support 10 of the FPSO type, having its bottom end resting on the sea bottom 13, and shown in three different positions 1a, 1b, and 1c;

FIG. 1A is a side view in section showing in detail the trench 12 that is dug by the foot 11 of the catenary during movements in which the pipe is lifted off and rested on the sea bottom;

FIG. 2 is a longitudinal section of a pipe and a side view of a peening robot 3 inside the pipe while it is being assembled, shown during peening treatment of the weld 6 between the ends of two pipe elements 2a and 2b, the weld being shown in the bottom half only of the section;

FIG. 2A is a section view of the pipe showing the peening robot 3 inside the pipe;

FIG. 3 is a longitudinal section view of one end of a pipe element showing a straight portion (root face) and an inclined portion (chamfer);

FIGS. 3A, 3B, 3C, and 3D are side views in section showing all or part of the respective ends of two pipe elements to be assembled together, respectively during an approach and positioning stage (3A), a welding stage (3B), an internal grinding stage (3C), and a peening stage (3D). FIGS. 3C and 3D show only a bottom portion of the weld so as to show more clearly the inside surface 6₃ of the weld bead 6 after it has been ground;

FIG. 3A′ shows a variant of FIG. 3A in the event of a small offset between the end root faces of two pipe elements for assembling together;

FIGS. 3B′ and 3C′ are fragmentary longitudinal sections corresponding to FIGS. 3B and 3C and showing only the bottom portion of the weld and of the pipe;

FIGS. 3E and 3F show variants of FIGS. 3B′ in the event that the pipe ends are offset, as in FIG. 3A′, with a incipient crack from the inside being shown at 2k in FIG. 3F;

FIG. 4A shows a pipe-laying ship fitted with a J-lay tower;

FIG. 4B is a side view of a pipe 2P being lowered down to the sea bottom and held under tension within said J-lay tower, and a string 2N held in the top portion of said J—lay tower, said string being approached to said suspended pipe 2P for the purpose of being assembled thereto by welding;

FIG. 4C is a side view in section showing the two ends of the pipe elements, in the bottom portion of the figure peening has not yet been performed at 7₂, while said peening is taking place in the top half-portion at 7₁;

FIG. 4D is a side view showing a string 2 made up of four pipe elements 2a-2d assembled to one another and ready for transferring to the J-lay ship of FIG. 4A;

FIG. 5 is a detail view of the peening tool 5;

FIG. 5A is a side view of a tiltable peening tool having a single row of projectiles; and

FIG. 6 is a detail view of a grinder tool 19 mounted on a said second carriage 4c, taking the place of the peening tool 5.

In FIG. 1, there can be seen a side view of a bottom-surface connection 1, 1a, 1b, and 1c of the SCR type, that is suspended from a floating support 10 of the FPSO type anchored at 15, the pipe resting on the sea bottom 13 at its point of contact 14a, 14b, 14c.

Curvature varies along the catenary from the surface, where the radius of curvature has a maximum value, to the point of contact where the radius of curvature has a minimum value R₀, R₁, R₂. Under the effect of waves, wind, and current, the floating support 10 moves, e.g. from left to right as shown in the figure, thereby having the effect of lifting or lowering the catenary-shaped pipe off or onto the sea bottom. In position 10c, the floating support is away from its normal position 10a, thereby having the effect of tensioning the catenary 1c and raising it, thereby moving the point of contact 14 towards the right from 14a to 14c; the radius of curvature at the foot of the catenary increases from R₀ to R₂, and the horizontal tension in the pipe generated at said point of contact also increases, and consequently the tension increases in the pipe and said floating support. In similar manner, when in position 10b, the movement to the right of the floating support has the effect of relaxing the catenary 1b and of resting a portion of pipe on the sea bottom. The radius R₀ at the point of contact 14a decreases to a value R₁, and similarly the horizontal tension in the pipe at the same time also decreases, as does the tension in the pipe at said floating support. This reduction in the radius of curvature at 14b gives rise to considerable internal stresses with the structure of the pipe, thereby generating fatigue phenomena that are cumulative and that can lead to the bottom-surface connection being destroyed.

Thus, the pipe presents a radius of curvature that is at its greatest at the top of the catenary, i.e. the point where it is suspended from the FPSO, and that decreases down to the point of contact 14 with the bottom 13. At this location, the radius of curvature at the suspended portion is at its smallest, however in the adjacent portion that is resting on the sea bottom, and assuming that said pipe is extending in a straight line, its radius of curvature becomes theoretically infinite. In fact said radius of curvature is not infinite but is very large, since, as a general rule, some residual curvature persists.

Thus, as explained above, as the floating support 10 moves on the surface, the point of contact 14 moves from right to left and in the region that is lifted off or rested on the bottom, the radius of curvature passes successively from a minimum value R_min to a value that is extremely large, or even infinite, in a configuration that extends substantially in a straight line.

This alternating flexing gives rise to fatigue phenomena that are concentrated within the foot region of the catenary, and the lifetime of such pipes is greatly reduced, and in general is not compatible with the lifetimes that are desired for bottom-surface connections, i.e. 20 to 25 years, or even more.

Furthermore, as shown in FIG. 1A, during these alternating movements of the point of contact, it is observed that the stiffness of the pipe, associated with the above-mentioned residual curvature, will over time dig a furrow 12 over the entire length that is raised and lowered, thereby creating a transition region in which a point of inflection 11 will exist, where the curvature changes direction in the transition regions, so as finally to reach an infinite value in the portion of the undersea pipe that rests in a straight line on the sea bottom, said portion being raised only exceptionally, e.g. in the event of the disturbing elements (swell, wind, current) acting on the floating support and on the catenary all accumulating maximally in the same direction, or else in the event of resonant phenomena appearing in the catenary itself. When the pipe rises, the point of inflection disappears and fibers that were previously in traction are put under compression, thereby creating a considerable amount of fatigue in this portion of pipe. Said fatigue is then one or two orders greater than the fatigue in the main section where no change of curvature occurs, and is incompatible with the looked-for lifetime of 25 to 30 years, or even more.

FIG. 4D shows a string 2 comprising four unit pipe elements 2a-2d that are assembled together by welds 2₂, 2₃, and 2₄ made in a workshop. The first end 2₁ of said string is for welding to the end 2₅ of already-assembled pipe that is being laid, with the end 2₅ of the string then constituting the new end 2₅ of the pipe being laid and being ready for assembly with the end 2₁ of the next string, assembly taking place on board the laying ship 8 shown in FIG. 4A, which ship is fitted with a J-lay tower 9. On board the ship, the strings are stored horizontally on deck, and then they are raised one after another by a pivoting ramp 18 from a horizontal position to a position in which they can be inserted in the J-lay tower 9. The already-laid portion of pipe 2P (not shown in FIG. 4A but visible in FIG. 43) is held under tension within the tower by means of a clamp. Thereafter, a new string 2N is lowered towards said pipe 2P that is held under tension, as shown in detail in FIG. 4B, and is finally welded thereto, and then subjected to the peening treatment of the invention, as shown in detail in FIG. 4C.

FIG. 2 is a section in side view showing two pipe elements 2a and 2b assembled end-to-end by welding 6 in a workshop, the top half-portion being shown in the approach stage prior to welding. Once the welding process has terminated and the weld has been subjected to quality control, a remotely-controlled device or robot 3 is inserted from the right-hand end of the right pipe 2b, said robot carrying a peening tool 5 of the invention and serving to position said peening tool astride said weld 6, substantially on the axis thereof. The robot 3 serves to enable the inside wall and the weld to be subjected automatically to peening treatment over a swath 7 of width L, e.g. having a total width of 2 centimeters (cm) to 6 cm, i.e. substantially 1 cm to 3 cm on either side of the weld bead 6.

FIG. 3 is a section showing the face of a pipe element that has machined in order to enable it to be assembled to the following element by welding. The face is machined in the plane perpendicular to the axis XX of the pipe and, towards the inside of the pipe, it presents a root face 16 occupying a few millimeters, generally 2 mm to 4 mm, followed by a chamber 17, e.g. a straight and conical chamfer as shown, or a curved and parabolic chamfer (not shown).

In FIG. 3A, two pipe elements have been positioned face to face, ready for welding. When the pipe elements present an extremely high level of quality, or when they have been made so as to present a diameter that is perfectly circular, the inside wall surfaces of said pipe element are substantially continuous. During welding (FIG. 3A, FIGS. 3B-3B′), this gives rise to a small internal projection 6₂ that is substantially uniform to the right (2k) and to the left (2h) and all around the periphery, as shown in detail in FIG. 3B′.

FIGS. 3E and 3F show the above-described unwanted phenomenon for this type of pipe that is to be subjected to fatigue over a period that may exceed 25 to 30 years. During welding performed from the outside by multi-head orbital welding robots, the first pass needs to merge perfectly with the respective root faces 16 of the two ends of the two pipe elements 2a and 2b. For this purpose, the chamfers 17 are prepared as shown in FIGS. 3 and 3A. It is the melting of said root faces that gives rise to a small amount of extra thickness in the form of a narrow seam or projection 6₂ (FIG. 3B) towards the inside of the pipe, said extra thickness being substantially rounded but presenting an irregular shape around the periphery of the inside wall of said pipe, and sometimes presenting an angular junction at the interface between the welding and the base metal of the inside surfaces 2i of the pipe elements.

In general, the pipe elements do not have an internal cross-section that is perfectly circular, with the section being slightly ovalized. Furthermore, wall thickness may vary around the periphery. Thus, when the ends of the two pipe elements for assembling together are placed face to face, although the alignment of FIG. 3A will be found at certain locations of the periphery, there will be other regions where there is an offset, as shown in FIG. 3A′. During the welding process, the projection 6₂, which is substantially symmetrical in FIG. 3B′, then presents unbalance, as shown in FIG. 3E. Thus, at 2h and 2k respectively identifying the transition region between the welding itself and the base metal of the pipe elements 2a and 2b, there exist respective angles α₁, α₂ between the tangents to the projection and the inside surface 2i of the pipe, which angles are open to a greater or lesser extent, as shown in FIG. 3E. In general, on the side set back inwards, on the left pipe element 2a in the drawing, the connection angle α₁ is small, whereas on the other element 2b, the connection angle α₂ is larger and may result in a sharp angle.

It is then in this region presenting sharp angles α₂ that there is a risk of generally-localized incipient cracks appearing under the effect of fatigue, which cracks initially propagate in the direction FF as shown in FIG. 3F, and finally propagate around the entire periphery of the pipe, thus leading to destruction of the weld and to destruction of the bottom-surface connection.

The welding process involves the use of heating and melting powers, and thus of considerable amounts of energy, since it is desirable to minimize cycle time, particularly for the welding that is performed on board the laying ship 8, as explained above with reference to FIG. 4A to 4D. Such pipe-installing ships have extremely high hourly operating costs, with welding and preparation operations constituting critical busy times. It is desirable to have welding process cycle times of the order of 10 minutes (min) to 12 min for pipes having a diameter of 300 mm and a thickness of 20 mm. The localized thermal shocks created by the power of the welding equipment are considerable and they give rise to residual regions of stress concentration that cannot be treated in conventional manner, in particular by thermal annealing, in order to obtain acceptable relaxation of stresses within a lapse of time that is compatible with the desired rates of laying. Said residual stresses may be compression stresses or traction stresses with traction stresses being more dangerous in terms of fatigue behavior over the lifetime of installations that may exceed 25 to 30 years or more.

During fatigue testing performed on lengths of pipe subjected to fatigue simulations corresponding to that which might be expected over a lifetime of 25 to 50 years, but actually carried out on a fatigue test bench, together with automated frequency spectrum and amplitude for the alternating stress cycles, the inventors have observed localized cracking phenomena at the interface between the base metal of a pipe element and the weld region, mainly where the root faces 16 melt and at the internal seam 6₂ of the weld bead 6. Because of localized quenching phenomena, combined with irregularities in local melting, weak points appear in which the material is in a residual traction stress state to significant level, generally in combination with the presence of a localized physical defect, such as angle. During movement of the pipe, it is specifically at such a location that incipient cracks will appear at 2_k, as shown in FIG. 3F, said cracks then propagating rapidly in radial and peripheral manner, in general in a direction FF through the thickness of the wall, thereby leading rapidly to destruction of the pipe and to unacceptable risks of pollution.

The device of the invention is constituted by a first carriage 3 having wheels 3e driven by a motor 3a and powered by an umbilical cord 3d. The wheels are connected to an axial main body 3₁ of the first carriage via a system of arms 3b mounted as a hinged parallelogram, preferably three parallelogram structures 3b, each carrying two wheels in alignment on the direction XX. The three parallelogram structures 3b are preferably uniformly distributed at 120° from one another, as shown in the cross-section of FIG. 2A, and they are actuated synchronously by springs or actuators 3c so that the main body 3₁ of the robot remains substantially on the axis XX of said pipe. The first carriage or robot 3 carries at its front end an axial shaft 4 that is movable in translation along the axis XX in a guide barrel 4a that is secured to the main body 3₁, passing axially therethrough, and movable in translation along said axis XX by an actuator (not shown) that may be a hydraulic cylinder or an electric motor, and that is preferably servo-controlled and operated by a computer via the umbilical cord 3d. Furthermore, said shaft 4 is capable of rotating about the same axis XX within said guide barrel 4a. Said rotation of the shaft 4 is actuated by an electric motor (not shown) incorporated in the main body 3₁, and preferably controlled and operated by said computer. At the front of the shaft 4, a guide support 4b secured to said shaft serves to support a second carriage 4c and guided in a direction perpendicular to the axis XX and to the inside wall 2i of the pipe 2. Said second carriage 4c carries a peening tool 5 that is secured thereto. Said peening tool is held in intimate contact with the inside wall 2i of the pipe 2, preferably with a constant bearing force, e.g. by means of a pneumatic actuator 4d, with said second carriage 4c being moved in a transverse direction. Thus, after the weld has been made and inspected, said robot 3 is inserted from the right-hand end of the pipe 2b, carrying the second carriage 4c that in turn carries the peening tool 5 in a retracted position, thus ensuring that the peening tool does not interfere with the inside surface of the pipe wall. Under drive from the motor 3a, the robot is moved to the weld 6 for treatment, under monitoring via a video camera 4e carried by the carriage 4c. The carriage is then locked in longitudinal position by locking its motor drive 3a and by increasing the pressure in the actuators 3c so as to cause the hinged arm 3b to pivot and jam the wheels 3a against the inside surface 2i of the wall of the pipes. The main body is then substantially on the axis XX of the pipe, and the position of the peening tool 5 is adjusted by acting on the position of the shaft 4 that is movable in translation along the axis XX, still under monitoring via the video camera 4e. The actuator 4d is then actuated so as to deploy the peening tool in a transverse direction in order to press it against the surface 2i of the wall of said pipe. The peening tool is then actuated while also driving the shaft 4 in rotation about its axis XX in order to apply said peening tool to the entire periphery of the inner weld bead together with the adjacent internal surfaces 2i of each of the pipe elements so as to form a peened swath 7 of width corresponding to the active width of said peening tool. The peening process is advantageously improved by performing successive circular passes of the peening tool that are slightly offset in longitudinal translation towards the left or towards the right, by modifying the longitudinal position of the shaft 4 that is movable in translation along the axis XX in the guide barrel 4a secured to the carriage 3.

A peening tool 5 is used that is of the kind described in FIGS. 6 and 7 of FR 2 791 293. More particularly, its vibratory surface is constituted by the end of a sonotrode. The metallic sonotrode is secured to a piezoelectric emitter via one or more acoustic amplifiers that, in known manner, present a profile that is adapted to amplify the amplitude of the oscillations of the sonotrode. The projectiles may be in the form of balls or in the form of spindles, or in the form of spikes. The ends of the projectiles strike the surface for treatment, and on impact their kinetic energy is transformed into plastic and elastic deformation energy, thereby creating or increasing the level of compression stress in the material at the point of impact.

FIG. 5 shows in greater detail a carriage 4c fitted with its peening tool 5 that is remotely controlled via an umbilical connection 5₃, the tool having a surface that vibrates under the effect of an ultrasound wave 5₂ in a transverse direction YY perpendicular to the longitudinal direction XX, thereby projecting the elongate projectiles 5₁ from a retracted position 5₁a to a deployed position 5₁b, with two successive such projectiles being spaced apart by a distance e lying in the range 2 mm to 5 mm.

Shifting the peening tool 5 in translation along the direction XX through a distance δx=e/5, for example, enables the surface for treatment to be peened between the point of impact of the various projectiles 5₁ when the peening tool is in a given longitudinal position, with this serving to peen the entire surface for treatment, and also to peen with insistence on some particular region, and/or to make peening uniform.

FIG. 5A shows a peening tool 5 having a single row of projectiles 5₁ in the direction XX. Said peening tool pivots about the axis 4f of the support 4g that is secured to the carriage 4c. The axis Y₁Y₁ of the projectiles 5₁, which also corresponds to the direction in which said projectiles 5₁ are projected against the surface for peening, is inclined at an angle β relative to said direction (YY) of radial movement in translation of the carriage 4c so as to arrive under the best conditions in the transition region 2h-2k as described above with reference to FIGS. 3B′ and 3F, i.e. substantially as close as possible to the direction perpendicular to the surface of the projection in said region. This makes it possible to peen with insistence on said transition regions 2h-2k in which unwanted cracking is liable to appear. Thus, a first peening tool as described with reference to FIG. 5 is used advantageously to perform general peening. Thereafter, peening is performed with insistence on each of the transition regions 2h-2k by means of said peening tool having a single row of projectiles, with both peening tools advantageously being installed on a common carriage 4c or on carriages that are independent and both secured to the same axial shaft 4.

This peening serves to provide local deformation over a controlled thickness as a function of the energy transmitted by the sonotrode to said needles, the metal of the weld, and the base metal at the end of each of the pipe elements. This plastic deformation of the metal makes it possible to establish a generalized and substantially uniform compression stress state throughout the treated region 7, thereby having the effect of absorbing any residual localized traction stress state that might result from the welding process and the above-described undesirable localized quenching phenomena.

Achieving compression depends on the power and the accuracy of the peening process, and it is generally performed over a thickness lying in the range 0.2 mm to 2 mm, thereby advantageously preventing unwanted incipient cracks from appearing.

The quality of the pipe in the welding region is advantageously improved by internally grinding 6₃ the weld prior to peening so as to eliminate geometrical surface defects, thereby enabling peening to be performed over an inside surface of the pipe and the welding that is substantially cylindrical where it is peened. Grinding is advantageously performed using a grinder tool 19 as shown in FIG. 6, which tool is mounted on a device similar to said above-described peening tool, but in which the peening tool is replaced with a grinder tool 19. The grinder tool 19 comprises a rotary grindwheel 19₁ that is mounted an a said second carriage 4c and that can therefore be moved in translation in the transverse direction YY such that the rotary grindwheel 19₁ comes to bear against the inside surface of the pipe and of the weld for grinding. At least one guide wheel 20 is securely mounted to the grinder tool 19 on one side thereof to serve as a guide to ensure that the rotary grindwheel 19₁ is held in position when it comes to bear against said inside surface of the pipe, i.e. so as to ensure that said rotary grindwheel 19₁ does indeed remain tangential to the bore of the pipe, and thus removes only the necessary quantity of the projection 6₂ of the weld bead 6, as shown in FIGS. 3C-3C′.

FIG. 6 shows a rotary grindwheel 19₁ of cylindrical shape and having an axis of rotation X₁X₁, which axis extends in a longitudinal direction parallel to the axial longitudinal direction XX of the pipe, the abrasive surface of the grindwheel corresponding to its cylindrical outer surface. In one embodiment, the cylindrical rotary grindwheel may extend in the direction X₁X₁ over a said distance L. Similarly, the guide wheel 20 presents an axis of rotation X₂X₂ in the longitudinal direction parallel to the axes XX and X₁X₁, such that the guide wheel 20 and the rotary grindwheel 19₁ present a common tangent X₃X₃ beside the inside surface 2i of the pipe, thus enabling the guide wheel 20 to guide the grinder tool by maintaining its axis X₁X₁ tangential to the bore 2i of the pipe, as described above.

FIG. 3D shows the state of the inside surface of the pipe in the peened region 7 of the inside surface of the weld over a width L that corresponds substantially to the width of the vibrating surface of the peening tool 5.

During prefabrication on land of the strings 2 as shown in FIG. 4D, the length of the unit elements 2a to 2d lies in the range about 6 m to 12 m, thereby making it necessary to insert the peening robot from the end that is closest to the weld for treatment, i.e. at a distance of about 6 m to 12 m depending on circumstances, and then cause the robot to travel along said distance in order to take up an accurate position astride said weld for treatment.

During on-site installation, the prefabricated strings generally have a length of about 50 m, as shown in FIG. 4D, or under certain circumstances of 25 m or of 100 m, and it is then necessary to make the robot travel over that distance in order to reach the welding region for treatment.

FIGS. 4A to 4C show two strings being assembled together together with the welding region being treated by peening, during on-site installation as performed on board a laying ship 8 that is fitted with a J-lay tower 9, as shown in FIG. 4A. For this purpose, the already-laid pipe element 2P is held securely in suspension from the foot of the tower, and a new pipe element 2N is transferred by means of a pivoting ramp 18 and in known manner from the horizontal position to the oblique position that corresponds to the inclination of the tower, after which it is positioned on the axis of the terminal suspended pipe element. Said pipe element 2N that is to be assembled is subsequently moved axially along the direction XX towards the suspended terminal pipe element 2P, as shown in FIG. 4B, and is then welded thereto in known manner. From the top end of the tower, the peening robot 3 is inserted into the pipe and lowered to the welding region that is situated 50 m below when using 50 m string, as shown in FIG. 4C, after which a swath 7 is peened in a manner similar to the treatment performed in a workshop and as described in detail with reference to FIGS. 2-2A. At the end of treatment, the peening robot is raised back to the top of the tower 9 and then the top end of the pipe is grasped and lowered towards the bottom of the tower so as to be ready to perform a new cycle of assembling and treating a new pipe string.

In the workshop, and on board the installation ship, at the end of the peening treatment of the welding region, it is advantageous to monitor the state of stresses in the treated region so as to ensure that traction stress states have been eliminated and replaced by compression stress states. The most appropriate inspection technique is the X-ray method that makes it possible to measure the inter-atomic distances within the surface of the material, and thus to characterize very accurately the stress state and level, regardless of whether stress is in traction, at rest, or in compression. Such means are available from the Applicant and are implemented using a robot similar to that described above, the peening tool 5 being replaced by the X-ray source and the associated sensors that are available from the supplier Stresstech (Finland). The signals recovered by the sensors are then sent to a signal processor unit, e.g. a computer, which deduces therefrom the real stress level that exists after and possibly also before the peening treatment of said welding region.

The present invention is described mainly for solving the problem associated with bottom-surface connections and more particularly in the region of the point of contact with the sea bottom in an SCR type connection. Nevertheless, the invention applies to any type of undersea pipe, whether it rests on the sea bottom, whether it is incorporated in a vertical tower, or indeed whether it constitutes a subsurface connection between two FPSOs, or between an FPSO and an unloading buoy.

The various types of subsurface connection are described in patent FR 05/04848 in the name of one of the Applicants, and more particularly in FIGS. 1A-1D and 2A. Said subsurface connections are particularly subject to fatigue phenomena when they are subjected to swell and to currents and above all to the movements of the floating supports, FPSO or loading buoy, which generates alternating stresses, particularly in the regions close to said floating supports.

Claims

1-21. (canceled)

22. A method of making steel undersea pipes, the method comprising end-to-end assembly by welding of unit pipe elements with welds having steel or metal alloy weld beads, the steel or metal alloy weld beads of said welds being located on the outside of the pipe, wherein localized peening is performed inside the pipe to increase the compression of the steel or metal in said welds and over the adjacent peripheral inside surface of the pipe on either side of the weld so as to create a surface swath that is peened over a limited distance L in the axial longitudinal direction XX of said pipe, preferably over a distance L that is not less than the width of the weld, on the inside of the pipe, plus a width of 1 mm to 10 mm on either side thereof.

23. The method according to claim 22, wherein the weld comprises a main weld bead at the outside of the pipe and an inside projection or seam of smaller thickness standing proud of the inside of the pipe.

24. The method according to claim 22, wherein said ends of unit pipe elements for welding together present, in longitudinal axial section, a straight end on the inside of the pipe forming a root face preferably occupying at least one-fourth of the thickness of the main portion of the pipe, and extended towards the outside of the pipe by a sloping chamfer.

25. The method according to claim 22, wherein material is removed by machining, preferably by grinding or by milling, from the inside surface of the pipe and from the weld bead over the surface that is to be peened, prior to said peening.

26. The method according to claim 22, wherein said peening is performed at least in the transition region between the inside surface of the weld bead and the adjacent inside surface of the pipe.

27. The method according to claim 22, wherein said peening is performed in such a manner as to establish compression or to increase compression over a thickness of 0.2 mm to 2 mm of said inside surface of the pipe and of said weld.

28. The method according to claim 22, wherein the limited distance L represents one to three times the thickness of the pipe.

29. The method according to claim 22, wherein peening is performed in such a manner as to obtain compression stress that is greater than 5 MPa, preferably greater than 50 MPa, over the entire peened surface.

30. The method according to claim 22, wherein said peening is performed with a peening device that is moved inside said pipe in translation and in rotation in the vicinity of said weld, the peening device comprising:

at least one peening tool mounted on a first motor-driven carriage;

said first carriage being suitable for moving inside said pipe in translation in the axial longitudinal direction XX of said pipe;

said first carriage supporting means for moving said peening tool in radial translation YY relative to said first carriage, enabling said peening tool to be applied against the inside surface of the pipe, or enabling the peening tool to be set back away from said inside surface of the pipe; and

means for moving said peening tool in rotation about said axial longitudinal axis XX of the pipe, enabling said peening to be performed over the entire circumference of the inside surface of said pipe one said rotation of the peening tool.

31. The method according to claim 30, wherein said peening tool comprises a vibratory surface that preferably extends over said limited distance L in the axial longitudinal direction XX, and a plurality of projectiles of rounded or pointed type suitable for being projected towards the inside surface for treatment by said vibrating surface in order to create a plurality of impacts.

32. The method according to claim 30, wherein said first carriage includes means for moving said peening tool in translation relative to said first carriage in said longitudinal axial direction XX.

33. The method according to claim 30, wherein:

said first carriage is moved in translation inside said pipe in said axial direction XX, such that said peening tool is substantially positioned so that it can perform peening in said weld region and on either side thereof over a said distance L for peening, in said longitudinal axial direction XX and astride said weld; then

said peening tool is moved against or close to the inside surface of the pipe by moving

said peening tool in radial translation YY; then

said peening tool is then moved in rotation about said axial longitudinal axis XX over the circumference of the inside surface of the pipe; and then where appropriate, the peening tool is moved in translation in the axial longitudinal direction XX relative to said first carriage so as to perform the peening and compression over the entire peened surface, in particular for a peening tool including a plurality of said projectiles of pointed or rounded type that are spaced about from one another.

34. The method according to claim 30, wherein:

said first motor-driven carriage supports a first shaft placed in said axial longitudinal direction XX; and

said first shaft supports a transverse guide support suitable for guiding the movement of a second carriage in radial translation in a transverse direction perpendicular to said axial longitudinal direction XX, and comprising means suitable for keeping said peening tool in position facing the inside surface of said pipe; and

said first shaft comprises means for driving it in controlled rotation about its said axial longitudinal axis XX, so as to enable said peening tool to be moved over the circumference of the inside surface of the pipe in controlled rotation about its axis; and

said first shaft is preferably suitable for being driven in translation relative to said first carriage in said axial longitudinal direction XX, over at least a said limited distance L.

35. The method according to claim 30, wherein said peening tool comprises a plurality of projectiles that are projected against said surface for peening from a vibrating surface of said peening tool.

36. The method according to claim 35, wherein said peening tool is mounted to pivot relative to said second carriage thus enabling the angle of inclination β of the projection direction Y1Y1 of said projectiles to be varied relative to said direction YY of movement in radial translation of said second carriage.

37. The method according to claim 30, wherein prior grinding is performed of the surface for peening with a grinder tool having a rotary grindwheel mounted in the place of or together with a said peening tool on a said first carriage.

38. The method according to claim 22, wherein said welding is performed using carbon steel, stainless steel, or a corrosion-resisting alloy of the Inconel type having high elasticity, and good fatigue resistance, and preferably Inconel of grade 625 or 825.

39. The method according to claim 22, comprising the following successive steps:

1) in a workshop on land, assembling the respective ends of at least two unit pipe elements together end-to-end by said welding in order to form pipe strings; and

2) at sea, on board a laying ship fitted with a J-lay tower, assembling respective ends of said strings together by said welding to form a pipe.

40. An undersea bottom-surface connection pipe including at least a portion having regions of said welds assembling together unit pipe elements, the welds being put into uniform compression by the method according to claim 22.

41. An undersea bottom-surface connection pipe according to claim 19, wherein it acts as an SCR catenary pipe having at least a portion that includes a region in contact with the bottom that extends above the bottom over at least 100 m, and preferably 200 m, said portion being assembled using a method according to claim 22.

42. A peening device suitable for use in a method according to claim 30, the device including at least one said peening tool mounted on a said first carriage suitable for moving in translation inside a pipe, the device comprising a said peening tool suitable for moving in longitudinal translation XX relative to said first carriage and in rotation about said axial longitudinal axis XX of the pipe in the vicinity of said welds, as defined in claim 30.